The present invention relates to a method for manufacturing a ferroelectric capacitor, and more particularly, to a method for manufacturing a ferroelectric capacitor to be used as a memory cell of a ferroelectric random access memory (FRAM).
Lead zirconium titanate (PZT; PbZr.sub.1-X Ti.sub.X O.sub.3) frequently used as a ferroelectric material has a considerably large polarization value and excellent electrical and material properties, though it has the disadvantage of poor durability. In a capacitor, fatigue is the phenomenon whereby the remanent polarization value is decreased by repeated switching and deterioration of ferroelectric polarization properties, exhibited in the deformation of the hysteresis loop of the capacitor, occurs.
In order to use a capacitor as a memory element, durability against switching test which is repeated to about 10.sup.12 times is required. However, in the case of a PZT-series capacitor using platinum (Pt) as an electrode, the fatigue phenomenon becomes increasingly serious after 10.sup.6 cycles and the ferroelectric properties are lost completely after 10.sup.9 cycles, and the function as a ferroelectric capacitor is last. A PZT-series ferroelectric capacitor using a ruthenium oxide (RuO.sub.X) electrode can be used for an extended period because of its excellent fatigue properties. In the case of a capacitor using RuO.sub.X electrodes, the PZT film is formed on the RuO.sub.X electrode by the sol-gel method. However, the sol-gel method has the problem of maintaining a uniform thickness of the PZT film. Also, the pure perovskite structure for the PZT film cannot be obtained, and therefore, it is substantially difficult to manufacture a practical ferroelectric capacitor with low leakage current. In the case of the capacitor using Pt electrodes, the PZT film is formed and then heat-treated, thereby producing the PZT film with a uniform thickness and a pure perovskite structure. Although the capacitor using the RuO.sub.X electrode adopts the same process as the capacitor using the Pt electrode, not only is it difficult to form a film of uniform thickness, but also a considerable amount of secondary pyrochlore structures appear. When the surface of the PZT film is examined with an optical microscope, a rosette structure of round spots with a radius of about 0.1.about.5 .mu.m are observed. It is considered that the rosette structures can be observed when not enough of perovskite nuclei are formed. Pb, a constituent element of PZT diffuses into the RuO.sub.X layer from the PZT film formed on the RuO.sub.X layer, which makes it difficult to form perovskite nuclei. In order to obtain a PZT film, the PZT film formed by spin-coating should be heat-treated for about 30 minutes in an oxygen atmosphere and at a temperature range from 600.degree. C..about.700.degree. C. or heated at a temperature of about 700.degree. C..about.850.degree. C. by a rapid thermal annealing (RTA) method. It is considered that the heat treatment process makes Pb in the PZT diffuse into the RuO.sub.X layer, thereby forming the rosette structure. Results of observing the rosette structure show that the rosette structure itself consists of pure perovskite structure. The boundary between the rosette structures is the mixture of pyrochlore and perovskite structures, and the charge accumulated in the capacitor may leak through such boundary surface. A capacitor with the RuO.sub.X /PZT/RuO.sub.X layered structure has a leakage current value of more than 10.sup.-5 .mu.A/cm.sup.2 which is more than ten times that of a capacitor with a Pt/PZT/Pt layered structure. Also, it is difficult to perform the etching process and forming a multilayered film due to the roughness of the PZT film's surface.